Ukrainian military vehicles near the eastern Ukrainian city of Slavyansk on June 3. / Andrew Kravchenko, AFP/Getty Images

by Oren Dorell, USA TODAY

by Oren Dorell, USA TODAY

Ukraine's president said Friday that he would be willing to resume peace talks with pro-Russian separatists on Saturday, as fierce fighting continued in east Ukraine and NATO launched naval exercises in the Black Sea.

However, Ukrainian President Petro Poroshenko said he had not heard back from the other parties.

Earlier in the week, Ukraine agreed in four-way talks with Russia, France and Germany to meet no later than Saturday to discuss the return of border checkpoints, the release of hostages and the laying down of arms by militants in east Ukraine.

Meanwhile, fighting appeared to intensify as Poroshenko's security forces continued an offensive against pro-Russian separatists that they resumed after a 10-day cease-fire expired Monday, according to media reports.

Three months of fighting has caused at least 200 civilian casualties - including 15 children - in the Donetsk and Luhansk regions. That is in addition to more 200 Ukrainian servicemen killed, according to the Kyiv Post.

Ukraine's National Security Council secretary, Andriy Parubiy, said that Ukrainian troops have recaptured more than a dozen eastern villages from pro-Russia separatists but that Russia is allowing the rebels to attack Ukrainian border posts from its territory, according to the Associated Press.

Russia has amassed 40,000 troops near the Ukrainian border, Parubiy said. That number could not be verified, the AP said. The 1,250-mile border is unmarked and unguarded outside of a few checkpoints.

Ukrainian National Guard spokesman Yurii Stets said 20 Russian tanks and 120 armored vehicles were spotted near Luhansk, where fierce fighting has taken place, according to Ukrainian news site Censor.net. Various reports described Ukrainian artillery being fired near Donesk and Luhansk, two cities seized by separatists in April.

Ukraine and the U.S. State Department say Russia is arming and supporting separatists in the east, a charge that Russia denies. Unrest there began after Russian President Vladimir Putin annexed Crimea in March. The rebels have asked to join Russia, and Russian nationalists have urged Putin to send troops into Ukraine. Western countries have threatened severe sanctions on Russia, and Putin has resisted those demands.

In the Black Sea on Friday, four NATO warships participated in annual military drills that involved fewer participants compared with previous years, according to Russia's state-owned news outlet RT.com.

Ukraine has hosted the exercises in the past, but most of its navy was seized in Russia's annexation of Crimea. The exercises this year include two ships from Italy and one each from Turkey and the United Kingdom. American and Greek ships are expected to join the group.